FIELD OF THE INVENTION
The present invention relates to a control unit and a control method of an automatic transmission used for a power transmission mechanism which converts a drive force of an engine in an automotive vehicle by means of the automatic transmission so as to transmit to an axle.
As the control unit of the automatic transmission in accordance with the prior art, for example, as described in Japanese Patent Unexamined Publication No. 6-147304, there has been known a method of performing a travelling load estimation and an incline estimation on the basis of a torque estimation so as to optimally control the automatic transmission. In this method, it is structured such as to determine an input torque of a torque converter by utilizing a characteristic of the torque converter when a velocity ratio (that is, a slip ratio) of the torque converter is equal to or less than a predetermined value, determine an engine torque by utilizing a torque characteristic of the engine in an area over the value, determine both calculated values, that is, calculate the input torque and the engine torque of the torque converter substantially at the same time when both are switched (that is, at a time of switching from a torque converter characteristic method to an engine torque characteristic method), calculate a difference between the both as a torque component of auxiliary machines (comprising loads of an air conditioner, a head light, a power steering and the like), and subtracting the torque component of the auxiliary machines from the calculated engine torque so as to determine an input shaft torque. Then, by multiplying this torque of the torque converter input shaft by a torque ratio obtained on the basis of another torque converter characteristic (a torque ratio characteristic), an output shaft torque in the torque converter is calculated.
However, since the torque converter characteristic (a characteristic of coefficient of a pump capacity and a torque ratio characteristic) uses a characteristic of an oil temperature (80.degree. C.) at which the engine and the automatic transmission are generally used, the characteristic of the torque converter changes under a state that an oil temperature before the engine is started is low and a state that an oil temperature becomes very high due to an overload operation, so that an error in calculating the torque of the torque converter output shaft is increased.
Then, for example, as described in Japanese Patent Unexamined Publication No. 8-121581, it has been known to correct the torque of the torque converter output shaft in accordance with the oil temperature of the automatic transmission.
However, in accordance with researches performed by the inventors of the present invention, it is not always sufficient to correct the torque of the torque converter output shaft on the basis of the oil temperature of the automatic transmission, and it becomes apparent there is a problem that the error of the calculated torque of the torque converter output shaft is still large.